U.S. Pat. No. 5,230,776 and U.S. Pat. No. 3,691,010 describe conventional machines with two presses, in both of which the drying cylinder acts as a counter roll and the first press has a suction press roll that can be crowned. To avoid running problems, the press felt must be conveyed out in a side loop between the two press nips. The web reaches the first press nip with a dry-solids content of about 10–15 percent. In a modified second embodiment of the machine in accordance with U.S. Pat. No. 5,230,776, the second press roll is disconnected and the web-carrying forming wire is extended to the first suction press roll while an open belt of wire type is substituted for the press felt to enclose the web between it and the forming wire, creating a sandwich construction that can be provided with airflow-generating means. Suction boxes can be arranged upstream of the sandwich construction in the loop of the forming wire. The known machine can thereby be modified from one operational embodiment to another that provides a higher dry-solids content, and thereby be adapted to different market needs. Thus, in the second embodiment, which is of a relatively complicated construction, the suction press roll is used in combination with an open belt and not solely with a press felt, and the suction means, located upstream of the suction press roll, are not arranged in a press-felt loop running through the press nip.
U.S. Pat. No. 4,055,461 describes a soft crepe paper machine having a pick-up suction roll and a press section with double press nips against a suction press roll, where the web is compressed in the first press nip before reaching the second press nip against the drying cylinder. Pressing the web in two press nips detrimentally affects the bulk of the paper.
So-called through-air drying can be used to increase the dry-solids content of the web before the drying cylinder. The increased dry-solids content allows the nip pressure to be reduced substantially in the subsequent press nip, in comparison with a conventional soft crepe paper machine, so that a desired high bulk is obtained. However, a through-air dryer represents high capital expenditure and requires additional space in the machine line. Furthermore, it has high running costs, as hot air is utilized.
In accordance with another technique for obtaining an increased dry-solids content before the press nip, a suction device is utilized and the pressing then takes place in an extended shoe press nip. A machine constructed in accordance with this technique is described in EP 0 926 296, which corresponds to U.S. Pat. No. 6,235,160. The press is a shoe press formed by a shoe press roll and a drying cylinder as the counter roll. The suction device is shown in the patent specification as a suction roll arranged before the extended shoe press nip at a distance from the drying cylinder. The suction roll enables the dry-solids content of the web to be increased to a high level before the press nip. Such an increased dry-solids content before the press nip is thus advantageous, as it becomes possible thereby to reduce the compression pressure in the press nip so that a substantially increased bulk can be obtained since the web is compressed to a lesser extent. However, a shoe press roll is a relatively complicated construction, entailing high capital expenditure and requiring special operational control.
Machines with similar shoe presses are described in EP 0 851 059 and EP 0 854 229, although, in both cases, a suction roll in the specified position before the shoe press nip is lacking.